Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 369

Figure 9.2 Normal EM appearance of the P. tetraurelia CVC. Centered around a radial
arm (ra) is the labyrinth of the smooth spongiome (ss) which is flanked by tubules of
the decorated spongiome (ds) and occasionally by a cisterna or the rough ER (rer) with
only few ribosomes attached. Unpublished micrograph. Bar¼0.1 mm.
2.2. Proton pump as a basic constituent
The V-type H þ -ATPase consists of two multimeric protein complexes, the
transmembrane V0 base piece and the catalytic V1 headpiece which can be
reversibly detached and reattached ( Hinton et al., 2009 ). The H þ -ATPase is
electrogenic and thus of central importance for CVC function. In Parame-
cium ( Grønlien et al., 2002 ), it has been shown to energize CVC membranes
for the—still hypothetic—secondary active transport of water and ions. First
discovered in Dictyostelium ( Fok et al., 1993; Heuser et al., 1993; Nolta et al.,
1993 ), its subunits have been identified successively in Paramecium ( Fok et al.,
1995; Wassmer et al., 2005, 2006 ). Here, the number of 17 a-subunits (SU)
is excessive when compared with the four isoforms occurring in the mouse
( Wassmer et al., 2005 ). This SU spans the holoenzyme and thus forms a con-
necting piece (stalk) between the V0 and the V1 part ( Wassmer et al., 2005 ),
as it does in all cells ( Lafourcade et al., 2008 ). Its polymorphy may allow for
the selective association of variable other SUs. Also, in Paramecium, one par-
ticular SU isoform, a2-1, is restricted to the CVC ( Wassmer et al., 2009 ).
This selectivity may reflect either a selective-membrane delivery process
in which other CVC components may participate and/or requirement